The invention relates to a reaction force mechanism for a brake power servo booster and particularly to a reaction force mechanism for a brake power booster of the type in which a master cylinder is provided on a rear side or on a brake pedal side of a power booster mechanism having a power piston.
Conventionally, in the above-described type of reaction force mechanism for a brake power booster, it has been proposed that the thrust of the power piston be transmitted as reaction force to an input shaft through a reaction disc. In addition, a hydraulic pressure reaction chamber, into which a hydraulic pressure generated in a pressure chamber in the master cylinder is led, is provided so that the hydraulic pressure in the reaction force chamber can be transmitted as the reaction force to an input shaft, etc. The latter case where the reaction force mechanism utlizes hydraulic pressure is advantageous in that the arrangement can be made compact because the master cylinder is disposed on the brake pedal side. However, it is desirable to have the reaction force mechanism function so as to prevent the reaction force from being transmitted in the very beginning of the braking operation so that the driver is prevented from sensing the loss of reaction force before the braking begins to take effect. Conventionally, the hydraulic pressure reaction force mechanism having such a function as mentioned above has a complicated construction and requires a relatively large number of parts.
Further, all the conventionally known hydraulic pressure reaction force mechanisms are directed to a master cylinder of a single system. Accordingly, if such a conventional hydraulic reaction force mechanism is applied to a master cylinder of two systems, the hydraulic pressure is led into a hydraulic pressure reaction force chamber from one of the two systems. Therefore, in case the one system is damaged to make it impossible to generate the hydraulic pressure, the reaction force cannot be transmitted to the driver in spite of the fact that the braking operation is actually effected through the other system.